The present invention relates to systems for measuring the characteristics of integrated circuits and other electronic devices and, more particularly, to systems for measuring differential signals used in conjunction with such devices.
Voltage measurements are commonly made by measuring the difference in potential between a conductor and a circuit's ground, which is often assumed to be at zero potential. While voltage is the difference between the electrical potentials at two nodes of a circuit and its measurement is, strictly speaking, the measurement of a differential signal, signaling that utilizes “ground” as the reference is referred to as “single ended” because the amplitude of the signal is represented by the difference between the ground potential and the potential in a single conductor.
On the other hand, a “differential” signal is transmitted on two conductors and the signal's amplitude is the difference between the electrical potentials in the two conductors or at two test points, neither of which is at ground potential. The potentials in the individual conductors, the signal and, ideally, its complement, commonly designated as + and −, vary around an average potential or signal, referred to as the common mode signal which may or may not remain constant. Differential signaling permits discrimination between smaller signal amplitudes because the recovery of the signal's value is largely independent of the value of the circuit's ground potential which may not be consistent within a system. In addition, differential signaling is relatively immune to outside electromagnetic interference and crosstalk from nearby signal conductors because the interference will likely produce an equal effect in each of the conductors of the differential signal. Any equal change in the potentials of the two conductors does not affect the difference between the potentials of the conductors and, therefore, the value of the differential signal. Differential signals also tend to produce less electromagnetic interference than single ended signals because changes in the signal level in the two conductors create opposing electromagnetic fields that tend to cancel each other out reducing crosstalk and spurious emissions. As a result of the inherent advantages in signal integrity, differential signaling has been adopted for electronic signaling at frequencies ranging up to microwave frequencies.
A probe provides the physical and electrical connections between a signal source or test points on a device-under-test (DUT) and an instrument for measuring the signal. For a probe to convey a signal between a device-under-test and an instrument while maintaining signal fidelity, the probe must have sufficient bandwidth, the continuous band of frequencies that the probe can pass without unacceptable diminishment of the signal's power, to pass the signal's major frequency components with minimum distortion. With the exception of DC signals having a frequency of 0 hertz (Hz) and pure sinusoidal signals having a single frequency, signals contain multiple frequencies having values that depend on the shape of the signal's waveform. In the case of square waves and other periodic signals, the bandwidth of the probe should be three to five times higher than the fundamental frequency of the signal to pass the fundamental frequency and, at least, its first few harmonics without undue distortion of their amplitudes. However, probes used for measuring differential signals are typified by bandwidth limitations and multiple probes are typically required to measure differential signals over the broad range of possible frequencies of such signals.
Probes for measuring differential signals comprise both active and passive types. An active probe typically includes a high performance differential amplifier as part of the probe's signal conditioning network. A differential amplifier amplifies the differential mode signal, the difference between the signal and the complementary signal which are the amplifier's inputs, and rejects the common mode signal, any signal that is common to both the signal and the complement. The output of the amplifier is referenced to ground to produce a single ended signal that is generally required by instrumentation used to measure differential signals. The bandwidth of active probes extends from DC up to approximately 15 GHz, the upper limit of operating frequency for high performance instrumentation amplifiers.
Passive AC probes are required when probing differential signals having higher frequencies than those transmissible with an active probe. An AC probe typically employs a common mode choke balun that introduces series inductance to the common mode signal path to attenuate the common mode signal and isolate the differential mode signal. However, the impedance of the common mode choke is frequency dependent and as the frequency of the differential signal decreases the common mode choke becomes less and less effective, producing no effect at DC. Common mode chokes with adequate bandwidth are difficult to build with impedances greater than 50 ohms and must be physically large for frequencies less than approximately 100 kilohertz (KHz). In contrast to the upper frequency limitation for active probes, passive AC probes have a lower frequency limit of approximately 10 KHz.
Differential signaling probes are typically expensive and, as a result of bandwidth limitations inherent in the types of probes used for measuring differential signals, multiple probes are required for testing devices utilizing differential signaling which may comprise signals having a broad range of possible frequencies. What is desired, therefore, is a probe having a bandwidth suitable for measuring differential signals comprising frequency components ranging from DC to microwave frequencies in excess of 100 gigahertz (GHz).